In a June 2009 interview, Bryan Singer admitted that he regretted declining to direct this film in favor of Superman Returns, confessing that he realized his mistake "before I was watching [the third [X-Men] film], during watching it, after watching it."

Famke Janssen did much research on Dissociative Identity Disorder (i.e. "split personality") in order to make her performance with Hugh Jackman in the medical lab more realistic.

[?] Mike Vogel, [?] Jed Bernard, and Nick Stahl all auditioned for the part of Warren Worthington/Angel.

Maggie Grace was originally cast as Kitty Pryde but was replaced with Ellen Page after the filmmakers discovered that she was too old for the role.

Ian McKellen (Eric Lensherr/Magneto) and Patrick Stewart (Professor Charles Xavier) had 20 years shaved off their features for the opening sequence. The filmmakers used digital technology to match current features to those in old photos. Stewart's "de-aged" effect in the opening scene was based on publicity photos from the first season of [Star Trek: The Next Generation].

The last scene with Magneto in Golden Gate park was actually filmed in London. It was originally meant to have Mystique sitting across from him, to imply that she had been on his side throughout the whole movie. Rebecca Romijn, however, was unavailable due to a television series ([Pepper Dennis]) she was filming at the time.

Summer Glau auditioned for the role of Kitty Pryde. She looked to Joss Whedon, who gave her a part in [Firefly] and Serenity, for advice because she knew he was a big [X-Men] fan, unaware that he had written the {Astonishing X-Men} comic book for Marvel, featuring the storyline about the mutant cure. Her audition script turned out to be pages from issue 5 of Joss Whedon's {Astonishing X-Men} book.

When Bryan Singer was still set to direct, he intended the Dark Phoenix storyline as the sole focus of the film. He also intended to feature the villain Emma Frost/White Queen as a major character, with the role intended for Sigourney Weaver. With Singer's departure, the character of Emma Frost was dropped, and the Dark Phoenix story was relegated to a subplot in favor of the "cure" storyline. He also planned on introducing long time fan favorite Gambit into the film, to serve as both the new recruit as well as a romantic rival of Iceman's for Rogue's affection. For the part, Singer had planned on casting Channing Tatum. However, once he left, the script was changed, and Gambit was reduced to a minor cameo, then ultimately dropped from the script.

Halle Berry had initially decided not to reprise her role as Storm for this film, citing lack of character development in the previous two installments and a tense relationship with director Bryan Singer. However, after Singer's departure and suffering a major box-office flop with Catwoman, Berry agreed to return on the condition that her role be expanded. Consequently, in this film Storm serves as leader of the X-Men, which is keeping with the comics since she served as team leader for a time in Xavier's absence.

The {Tracking Mutations} issue of [Scientific American] that Hank McCoy/Beast is reading in his office while hanging from the ceiling was not a fake prop; the magazine actually had that cover story in October of 2005.

Angel's wings were initially too heavy for Ben Foster, and were remade from foam.